The WordPress ecosystem is increasingly hostile to commercial developers.
As many are fond of saying in the open-source world, if you don't like the license terms, you are free to make your own [software] under any license you like, provided your code is your own, or licensed under terms compatible with the license you choose.
Envato's stance enables them to enforce copyright terms, whereas a GPL license is, by nature, far more open and encourages reuse by others. It's my opinion that WordPress is in the right here (aside from the fact that it's their convention to do with what they please) regarding license enforcement, but I can certainly understand Envato's position.
Well, I'd be careful with that. The open source project I contribute to (KDE) has quite a bit of GPL'ed code, but it also has quite a bit of other licenses in use, so it is certainly possible to have arbitrary licenses that make up a given single derivative work.
If I understand Envato's argument correctly, they make derivative themes (and license the derivation under GPL), which has embedded hooks to allow for other content to be read in later. This other content (normally art or CSS?) is not necessarily licensed under GPL, because it is not considered by Envato to be derivative of WordPress content.
To be honest I would side with Envato here. Linking khelpcenter (GPL) to kdelibs (LGPL) does not make kdelibs suddenly GPL'ed software, and using khelpcenter to display GFDL'ed documentation in an integrated fashion doesn't make khelpcenter suddenly GFDL'ed.
The real question is whether the "additional content" is truly derivative or whether it is originated by Envato and then linked in (since GPL has provisions to allow linking in content with other licenses). Of course, maybe WordPress used a modified license to forbid linking, but then that wouldn't really be GPL, it would be something else.
Yes, so long as those other license are GPL-compatible. 3-clause BSD, MIT, ISC, etc. are all GPL-compatible. The license that Envato is allowing or using for the non-PHP theme assets is not, so the question does become one of whether the theme assets are a derivative work.
That's not GPL people, that's WordPress people picking the wrong license. They shouldn't be using GPL if they want that.